Facile Fabrication of Ultralow‐Density Transparent Boehmite Nanofiber Cryogel Beads and Their Application to a Nanoglue

Abstract Ultralow‐density transparent porous bulk materials with densities of 5 mg cm⁻³ or less have only been obtained by supercritical drying. In this work, a simple method is reported for obtaining ultralow‐density transparent porous beads with a diameter of several millimeters. In the method, a boehmite nanofiber dispersion (BNF sol) is dripped directly into liquid nitrogen and the beads are vacuum‐dried. Normally, porous materials obtained via freeze drying exhibit strong light scattering due to structural nonuniformity and are not transparent. However, using dilute BNF as a skeleton structure suppressed the inhomogeneity due to aggregation during the dispersion medium crystallization. The lowest bulk density of the cryogel was 1.1 mg cm⁻³, and the visible light transmittance reached 90 %. By dispersing functional materials into the BNF sol before freezing, composite materials were obtained. The facile method can be expected to develop the field of functional porous materials.